DESCRIPTION: (Applicant's Abstract) Susceptibility to infection, viral persistence and viral clearance are functions of a dynamic relationship between virus and host. Our understanding of the mechanisms of these interactions in HCV are limited. Because of the lack of an available animal model for study of HCV immunology, prior studies have depended largely on study of lymphocytes removed from infected individuals, correlating these in vitro findings with clinical and virological outcomes. We will apply newly developed tetramer technology to study the immune response to HCV in injection drug users: (i) those with resolved HCV infection (seropositive confirmed by RIBA in the absence of viremia), (ii) those with persistent HCV, infected with one major viral variant, and (iii) those with persistent HCV, infected with multiple HCV variants. This proposal takes advantage of an established cohort of HCV-seropositive injection drug users with and without HIV infection. It represents collaborations between clinicians, epidemiologists, immunologists and cell biologists at the San Francisco and Palo Alto VAs and UCSF and Stanford Universities. We propose to use heteroduplex and phylogenetic analysis of HCV to measure the evolution of HCV variants. Our primary hypothesis is that the immune response to HCV (HCV-specific tetramer positive CTLs with an activated phenotype, CD 69+) is present in those who clear virus, but not in those who have persistence of a single infection. Our secondary hypothesis is that in those with ongoing drug use who are infected with multiple HCV variants, the immune response is more effective in those who have sequential infection with new variants in comparison with those who have infection with multiple variants simultaneously. The third part of this proposal, will examine the hypothesis that HCV-specific activated T cells (CD69 positive) are reduced in those with HCV/HIV coinfection compared to those with HCV infection alone, despite an overall increase in CD8 activity in the former group. Finally, with the establishment of this cohort, we will be able to store serum and lymphocytes for future analysis of genetic determinants of viral persistence/clearance.